A significant gap exists in understanding the mechanism of contractile-ring assembly during cytokinesis, which partitions cellular constituents into two new daughter cells and plays a crucial role in cell reproduction and cell differentiation. The long-term goal of our research is to investigate cytokinesis in yeast, in normal somatic and stem cells, as well as in cancer cells. The objective of this application is to investigate the molecular mechanism of the assembly of the contractile ring in fission yeast cytokinesis. The fission yeast Schizosaccharomyces pombe has emerged as one of the leading model systems for the analysis of cytokinesis. Not only is it genetically tractable and favorable for microscopic analysis, but it also has the smallest fully sequenced eukaryotic genome and carries out cytokinesis much like animal cells. Contractile rings consisting of actin filaments, myosin-II motors, and >30 other proteins are essential for cytokinesis in both yeast and animal cells, including humans. The majority of these proteins are conserved during evolution. The central hypothesis of this proposal is that the cytokinetic contractile ring assembles progressively at the cleavage site from a broad band of precursor nodes into a complex protein structure. Actin cross-linking proteins and Polo kinase play essential structural and signaling roles during the assembly. Based on a solid foundation of background research and strong preliminary data, this hypothesis will be tested by investigating three specific aims: 1) Elucidate the roles of the anillin Mid1p in node formation and identify its binding partners in the nodes; 2) Establish the roles of actin cross-linking proteins Fimbrin and 1-actinin in coalescing the nodes into the contractile ring; 3) Explore the signaling pathways that trigger node formation and node condensation into the contractile ring. A combination of cellular, genetic, biochemical, and confocal microscopic approaches will be employed in these studies. This research is significant because it will reveal how proteins work together during contractile-ring assembly in cytokinesis, and how the anillin Mid1p initiates the assembly of the contractile ring by interacting with evolutionarily conserved structural and signaling proteins. Discerning the assembly of the essential contractile-ring is an important step towards understanding cytokinesis. Additionally, it will help us understand complex actomyosin contractile systems in other cellular processes.